Hybrid powertrains typically employ an engine and an electric motor operatively connected to a transmission. The engine is stopped and restarted under certain conditions in order to improve fuel economy. When the engine is off, it is desirable to keep the transmission engaged in order to reduce the delay time to put power to the wheels when the driver commands it. Automatic transmissions require hydraulic pressure to be applied to clutches in order to maintain engagement. The hydraulic pressure is provided by an oil pump.
In prior art transmissions, the oil pump is driven by the engine. With the engine off, an alternative means to supply pressure to the clutches must be provided. Some prior art hybrid powertrains include an auxiliary pump driven by the electric motor to supply hydraulic pressure to clutches to maintain transmission engagement. A valve controls the hydraulic pressure so that it is provided either solely from the engine-driven pump or solely from the auxiliary pump. Accordingly, the main pump must be sized sufficiently to provide adequate pressure to maintain clutch engagement when the engine is at low speed.